Merge pull request #226 from nagisa77/codex/update-username-validation-rules

Adjust username validation and signup UI

src/main/java/com/openisle/service/UsernameValidator.java

@@ -14,8 +14,8 @@ public class UsernameValidator {
      * @param username the username to validate
      */
     public void validate(String username) {
-        if (username == null || username.length() < 6) {
-            throw new FieldException("username", "Username must be at least 6 characters long");
+        if (username == null || username.isEmpty()) {
+            throw new FieldException("username", "Username cannot be empty");
         }
     }
 }

src/test/java/com/openisle/service/UsernameValidatorTest.java

@@ -0,0 +1,22 @@
+package com.openisle.service;
+
+import com.openisle.exception.FieldException;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class UsernameValidatorTest {
+
+    @Test
+    void rejectsEmptyUsername() {
+        UsernameValidator validator = new UsernameValidator();
+        assertThrows(FieldException.class, () -> validator.validate(""));
+        assertThrows(FieldException.class, () -> validator.validate(null));
+    }
+
+    @Test
+    void allowsShortUsername() {
+        UsernameValidator validator = new UsernameValidator();
+        assertDoesNotThrow(() -> validator.validate("a"));
+    }
+}